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 for Virtual focus
From The Collaborative International Dictionary of English v.0.48 :

  Virtual \Vir"tu*al\ (?; 135), a. [Cf. F. virtuel. See Virtue.]
     1. Having the power of acting or of invisible efficacy
        without the agency of the material or sensible part;
        potential; energizing.
        [1913 Webster]
              Heat and cold have a virtual transition, without
              communication of substance.           --Bacon.
        [1913 Webster]
              Every kind that lives,
              Fomented by his virtual power, and warmed. --Milton.
        [1913 Webster]
     2. Being in essence or effect, not in fact; as, the virtual
        presence of a man in his agent or substitute.
        [1913 Webster]
              A thing has a virtual existence when it has all the
              conditions necessary to its actual existence.
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              To mask by slight differences in the manners a
              virtual identity in the substance.    --De Quincey.
        [1913 Webster]
     Principle of virtual velocities (Mech.), the law that when
        several forces are in equilibrium, the algebraic sum of
        their virtual moments is equal to zero.
     Virtual focus (Opt.), the point from which rays, having
        been rendered divergent by reflection of refraction,
        appear to issue; the point at which converging rays would
        meet if not reflected or refracted before they reach it. 
     Virtual image. (Optics) See under Image.
     Virtual moment (of a force) (Mech.), the product of the
        intensity of the force multiplied by the virtual velocity
        of its point of application; -- sometimes called virtual
     Virtual velocity (Mech.), a minute hypothetical
        displacement, assumed in analysis to facilitate the
        investigation of statical problems. With respect to any
        given force of a number of forces holding a material
        system in equilibrium, it is the projection, upon the
        direction of the force, of a line joining its point of
        application with a new position of that point indefinitely
        near to the first, to which the point is conceived to have
        been moved, without disturbing the equilibrium of the
        system, or the connections of its parts with each other.
        Strictly speaking, it is not a velocity but a length.
     Virtual work. (Mech.) See Virtual moment, above.
        [1913 Webster]

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